1. Field of the Invention
The present invention relates to a bottom structure of a thin-walled can for sealingly enclosing liquid or gas.
2. Description of the Prior Art
A bottom structure of a representative can in the prior art is illustrated in cross-section in FIG. 7. Such can bottom structure generally consists of a dome section 1, a counter section 2, a ground section 3, a heel section 4, and a side wall section 5. By the way, cans sold currently in the market are generally classified into the following three types of shapes:
______________________________________ Shape of Can Bottom Portion Shape of (Counter Section, Ground Type Dome Section Section & Heel Section) ______________________________________ 1 Spherical V-shape 2 Flat V-shape 3 Spherical C-shape ______________________________________
Cross-sectional shapes of representative bottom structures of the above three types are respectively illustrated in FIG. 7 (Type-1), FIG. 8 (Type-2) and FIG. 9 (Type-3). It is to be noted that the can of type-3 having a spherical dome section and a C-shaped bottom portion is of old-fashioned type and at present cans tend to be of type-1 or type-2.
The cross-sectional shapes of the bottom plates of cans in the prior art have an abrupt transition point of curvature. More particularly, an abrupt transition point of curvature in FIG. 7 is point A, where a radius of curvature R.sub.D of a spherical surface of the dome section changes abruptly to a radius of curvature r.sub.1 of a corner with the counter section. Abrupt transition points of curvature in FIG. 8 are also present at point B in addition to a location corresponding to point A in FIG. 7, the point B being a point on an intersection line between a flat plane and a conical surface, where the cross-sectional curve bends sharply. With regard to the cross-sectional shape of the bottom plate of the old-fashioned type of can shown in FIG. 9, also an abrupt transition point of a curvature is present, though not specifically indicated. Therefore, the bottom structure of the can in the prior art involved the problem that if an inner pressure should act upon the inner surface of the can, local concentration of stress would arise at the abrupt transition point of curvature, resulting in plasticization of the can wall at that portion. Hence, the support for the dome section (bottom plate) would be deteriorated, and pressure-proofness of the can would be lowered.
In addition, for the purpose of smoothly transmitting a pressure acting upon the dome section 1 to the ground section 3, it is effective to select a counter-sink angle .theta. (the angle formed between the cross-section of the counter section 2 and an axial or vertical line) to be small. However, whether with a spherical dome or a flat dome, in order to select the counter-sink angle .theta. small a corner having a small radius of curvature must be provided. However, this would form the above-described abrupt transient point of a curvature. Thus, there is the problem that even if it is attempted to improve pressure-proofness of a can by reducing the counter-sink angle .theta., such attempt will not be effective.